linux-ext4 - Re: [PATCH 07/12] fs: stop using get_super in fs_mark

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Message-ID: <20230803131223.qkxsxs7svtcu5buz@quack3>
Date:   Thu, 3 Aug 2023 15:12:23 +0200
From:   Jan Kara <jack@...e.cz>
To:     Christoph Hellwig <hch@....de>
Cc:     Al Viro <viro@...iv.linux.org.uk>,
        Christian Brauner <brauner@...nel.org>,
        Jan Kara <jack@...e.cz>, Chris Mason <clm@...com>,
        Josef Bacik <josef@...icpanda.com>,
        David Sterba <dsterba@...e.com>, Theodore Ts'o <tytso@....edu>,
        Andreas Dilger <adilger.kernel@...ger.ca>,
        Jaegeuk Kim <jaegeuk@...nel.org>, Chao Yu <chao@...nel.org>,
        Ryusuke Konishi <konishi.ryusuke@...il.com>,
        "Darrick J. Wong" <djwong@...nel.org>,
        Jens Axboe <axboe@...nel.dk>, linux-btrfs@...r.kernel.org,
        linux-ext4@...r.kernel.org, linux-f2fs-devel@...ts.sourceforge.net,
        linux-nilfs@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-xfs@...r.kernel.org, linux-block@...r.kernel.org
Subject: Re: [PATCH 07/12] fs: stop using get_super in fs_mark_dead

On Wed 02-08-23 17:41:26, Christoph Hellwig wrote:
> fs_mark_dead currently uses get_super to find the superblock for the
> block device that is going away.  This means it is limited to the
> main device stored in sb->s_dev, leading to a lot of code duplication
> for file systems that can use multiple block devices.
> 
> Now that the holder for all block devices used by file systems is set
> to the super_block, we can instead look at that holder and then check
> if the file system is born and active, so do that instead.
> 
> Signed-off-by: Christoph Hellwig <hch@....de>

Looks good. Feel free to add:

Reviewed-by: Jan Kara <jack@...e.cz>

								Honza

> ---
>  fs/super.c | 30 ++++++++++++++++++++++++++----
>  1 file changed, 26 insertions(+), 4 deletions(-)
> 
> diff --git a/fs/super.c b/fs/super.c
> index 09b65ee1a8b737..0cda4af0a7e16c 100644
> --- a/fs/super.c
> +++ b/fs/super.c
> @@ -1209,17 +1209,39 @@ int get_tree_keyed(struct fs_context *fc,
>  EXPORT_SYMBOL(get_tree_keyed);
>  
>  #ifdef CONFIG_BLOCK
> +/*
> + * Lock a super block that the callers holds a reference to.
> + *
> + * The caller needs to ensure that the super_block isn't being freed while
> + * calling this function, e.g. by holding a lock over the call to this function
> + * and the place that clears the pointer to the superblock used by this function
> + * before freeing the superblock.
> + */
> +static bool lock_active_super(struct super_block *sb)
> +{
> +	down_read(&sb->s_umount);
> +	if (!sb->s_root ||
> +	    (sb->s_flags & (SB_ACTIVE | SB_BORN)) != (SB_ACTIVE | SB_BORN)) {
> +		up_read(&sb->s_umount);
> +		return false;
> +	}
> +	return true;
> +}
> +
>  static void fs_mark_dead(struct block_device *bdev)
>  {
> -	struct super_block *sb;
> +	struct super_block *sb = bdev->bd_holder;
>  
> -	sb = get_super(bdev);
> -	if (!sb)
> +	/* bd_holder_lock ensures that the sb isn't freed */
> +	lockdep_assert_held(&bdev->bd_holder_lock);
> +
> +	if (!lock_active_super(sb))
>  		return;
>  
>  	if (sb->s_op->shutdown)
>  		sb->s_op->shutdown(sb);
> -	drop_super(sb);
> +
> +	up_read(&sb->s_umount);
>  }
>  
>  static const struct blk_holder_ops fs_holder_ops = {
> -- 
> 2.39.2
> 
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR